In a long history of moviegoing, I’ve seen few films as harrowing to watch as Joshua Oppenheimer’s 2012 documentary, “The Act of Killing.” The director’s follow-up, “The Look of Silence,” is nearly as powerful, but marginally less horrifying, at least for those who’ve seen the first film — they know what to expect. The subject is the killing of from half a million to a million Indonesians in 1965-66, following an attempted coup that eventually led to the ouster of President Sukarno.